You can use insertBefore()
:
import xml.dom.minidom
data = """<node-a>
<node-1/>
<node-2/>
<node-3/>
</node-a>"""
dom = xml.dom.minidom.parseString(data)
node_a = dom.getElementsByTagName('node-a')[0]
node_4 = dom.createElement('node-4')
node_a.appendChild(node_4)
node_3 = dom.getElementsByTagName('node-3')[0]
node2_a = dom.createElement('node-2a')
node_a.insertBefore(node2_a, node_3)
print dom.toprettyxml()
prints:
<node-a>
<node-1/>
<node-2/>
<node-2a/>
<node-3/>
<node-4/>
</node-a>